Yes, K/DA is a League of Legends virtual band impersonated by Ahri, Evelynn, Kai'Sa and Akali — but as you might expect their songs are written and interpreted by real musicians and singers working for Riot Games. Released in November 2018, their first song POP/STARS was a global success and has been since listened more than 200 million times.
A few days ago, a Twitter user discovered some lyrics for two new songs that apparently have yet to be released...
If we take into account the fact that Worlds officially start on October 2, it seems unlikely that these songs were written for the opening ceremony. However, it's not surprising to see Riot producing more K/DA content since it was a huge success. Also, the seeming release date of these two new songs perfectly coincides with the release date of POP/STARS.
If the information were to be confirmed, we could probably hope for a small K/DA show during the final phases of Worlds...